with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows several overlapping peaks
with a total duration of about 20 sec. The peak count rate
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 22:43:07.3 UT, 78.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 82.9666, -5.3883 which
is equivalent to:

RA(J2000) = 05h 31m 51.99s
Dec(J2000) = -05d 23' 18.0"

with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 27 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density.
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.24.
We note that this event is in the vicinity of the Orion Nebula.
However, it appears in all other respects to be a typical GRB.
Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti AT brera.inaf.it).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

with an uncertainty of 1.7 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular #18109
P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), T.G.R. Roegiers (PSU), L.M.
McCauley (PSU), J.A. Kennea (PSU), B.P. Gompertz (U. Leicester), J.P.
Osborne (U. Leicester), A. D'ai (INAF-IASFPA) and B. Sbarufatti
(INAF-OAB/PSU) report on behalf of the Swift-XRT team:
We have analysed 12 ks of XRT data for GRB 150801B (Sbarufatti et al.
GCN Circ. 18104), from 64 s to 40.0 ks after the BAT trigger. We have
analysed 12 ks of XRT data for GRB 150801B (Sbarufatti et al. GCN Circ.
18104), from 64 s to 40.0 ks after the BAT trigger. The data comprise
8 s in Windowed Timing (WT) mode while Swift was slewing to the burst,
270 s in PC mode, a further 185 s in WT mode during a bright flare
starting at T0+366 s (also seen in the BAT), with the remainder 11.5 ks
in Photon Counting (PC) mode. The enhanced XRT position for this burst
was given by Evans et al. (GCN Circ. 18108).
The late-time light curve (from T0+4.9 ks) can be modelled with a
power-law decay with a decay index of alpha=1.74 (+0.18, -0.17).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.79 (+0.21, -0.19). The
best-fitting absorption column is 3.0 (+/-0.4) x 10^22 cm^-2, in
excess of the Galactic value of 2.7 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 7.6 x 10^-11 (1.5 x 10^-10) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:

If the light curve continues to decay with a power-law decay index of
1.74, the count rate at T+24 hours will be 1.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (2.2 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00650977.
This circular is an official product of the Swift-XRT team.

with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 62%.
The mask-weighted light curve shows two widely separated episodes. The first
episode was about 40 seconds long starting at about T-20 seconds. There were two
prominent overlapping FRED peaks at T0 and T+10 seconds, followed by a return to
background level. The second episode became detectable in BAT at about T+330
seconds and returned to background about T+450 seconds. There were two smaller
cusp-shaped peaks at T+370 and T+390 seconds, about 10 and 7 seconds long. The
peaks between 330 and 450 seconds are at the same time as bright X-ray flares
detected by XRT. T90 (15-350 keV) was 426 +- 30 sec (estimated error including
systematics).
The time-averaged spectrum from T-30.16 to T+433.94 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.84 +- 0.14. The fluence in the 15-150 keV band is 3.1 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+9.32 sec in the 15-150 keV band
is 2.5 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/650977/BA/